Well. In a roundabout way it is Angel's fault. It is his responsibility. Had he not turned Drusilla, William would have continued writing bad poetry and pining after Cecily or whatever her name was. It's why the term "sire" doesn't have "great" or "grand" added to it with further generations. In the vampire mystique there's no real distinction.

That, and Whedon bungled years ago with the whole Angel/Spike/Darla/Drusilla history thing. But he recovered. Kinda. I mean it's easy to understand why some journalists would easily get confused. Just so long at Mutant Enemy doesn't make such a faux pas in the actual series, where Spike says to Angel something like, "you sucked blood outta me and made me a vampire, man! You're my Yoda!" That would be hard to take back.

He called Angel his sire twice. In "School Hard" and "In the Dark" on AtS. That was before the Fool for Love flashbacks, so a lot of people took it literally. Joss explained the idea of vampiric family lines, but there were a lot of canon police that went psycho over FFL.

Well, if Angel is Spike's sire, Darla is more Spike's sire as she not only turned Angel but encouraged him to turn Drusilla, and then encouraged Drusilla to take a mate of her own, Spike.
Not that I really care. The sooner Spike gets staked or turned bad again, the sooner the show will get better again. Spike with soul? That's like Aretha Franklin with Country & Western